1. Field of the Invention
The present invention relates generally to apparatus and methods for drying and deagglomerating substances comprised of finely-divided solids suspended in a fluid medium, and it more specifically relates to apparatus and methods for processing wet, pasty, sticky substances, such as municipal sludge, into a workable, powdered product.
2. Description of the Related Art
The primary problem currently facing every municipal wastewater treatment facility is the cost-effective, energy-efficient and environmentally-sound disposal of sludge, the end product of wastewater processing. North America produces about two pounds of sludge daily for every man, woman and child. In the United States, there are currently approximately 35,000 wastewater treatment plants, with an estimated 2,500 slated for construction within the next 10 years to accommodate the growing population base. The EPA currently estimates that approximately 15,000 of these are privately-owned, and that these alone generate 110-150 million wet tons of sludge cake per year. Annual costs for processing this waste are in excess of five billion dollars.
Historically, municipal sludge management disposal programs relied upon: land filling; surface disposal; incineration; ocean dumping (banned in the United States in 1992); and/or, beneficial reuse through land application. Each of these methods can be relatively inexpensive, but each can have undesirable aspects which result in a negative long-term cost to the environment.
The major portion of the United States' sludge has been sent to landfills. As a result, valuable landfill space for large communities has been either so reduced, or the quality of the sludge cake is so undesirable, that a great proportion of the nation's sludge is now hauled a considerable distance overland before disposal. For example, New York City sends 20% of its sludge output to the range lands of Texas. Portland, Oreg. trucks approximately 75,000 tons per year over 200 miles to Hermiston, Oreg. to be land-applied for beneficial reuse. Although land-application for beneficial reuse has heretofore been the best current alternative, even this has drawbacks, including substantial fuel and personnel costs, and wear-and-tear on roads.
Another substantial portion of the waste in the United States has been sent to surface disposal sites. Surface disposal sites include monofills; surface impoundments and lagoons; waste piles; dedicated disposal sites; and, dedicated beneficial use sites.
However, a large number of states now have site restrictions or management practices governing sludge disposal. And federally, the Clean Air Act, which governs sludge incineration and the disposal of its residual ash, has recently been amended (the "Clean Air Act Amendments") to levy stricter air emission measures for incineration. Further, in 1993 the EPA published the 40 CFR part 503 Sludge Regulations (503 Regulations) employing the EPA's "exceptional quality" sludge program. The 503 Regulations established "Standards for the Use or Disposal of Sewage Sludge" applicable to all wastewater treatment facilities. The 503 Regulations establish requirements for the final use or disposal of biosolids when biosolids are: applied to land to condition the soil or fertilize crops or other vegetation grown in the soil; when they are placed on a surface disposal site for final disposal; or, when they are fired in a biosolids incinerator. The 503 Regulations also direct that if biosolids are placed in a municipal solid waste landfill, they must meet the provisions of 40 CFR Part 258 which covers, in great detail, all aspects of establishing, maintaining and monitoring such landfills. In light of the foregoing, almost all communities are pursuing alternatives to incineration and landfilling.
"Land application for beneficial use" is the application of biosolids to land, either to condition the soil, or to fertilize crops or other vegetation grown in the soil. Biosolids can be beneficially land-applied on agricultural land, forest land, reclamation sites, golf courses, public parks, roadsides, plant nurseries and home land and gardens. Under the 503 Regulations, biosolid products that meet stringent requirements, including sufficiently low concentrations of certain pathogens and pollutants, and minimal attractiveness to disease vectors such as insects and rodents, are considered by the EPA to be Class A, "Exceptional Quality" biosolids. Class A biosolids are treated by the EPA in the same manner as common fertilizers; thus, they are exempt from federal restrictions on their agricultural use or land application. Biosolids falling short of the highest EPA standards may nevertheless qualify as Class B biosolids.
Biosolids that meet Class B requirements may also be applied to the land for beneficial use, but are subject to greater record keeping, reporting requirements and restrictions governing, among other items, the type and location of application, and the volume of application. Biosolids applied to the land for agricultural use must meet Class B pathogen levels and, if applied in bulk, require an EPA permit.
The 503 Regulations subject surface disposal to increased regulation by requiring, among other things: restricted public access; run-off and leachate collection systems; methane monitoring systems; and, monitoring of, and limits on, pollutant levels. In addition, sludge placed in a surface disposal site is required to meet, at least, Class B requirements.
Over the past few years, there has been a movement toward surface disposal of wet sludge. However, in large metropolitan areas and rural communities alike, proposals to land-apply wet sludge have been met with great resistance from the public. Factors affecting the acceptance of land-application are local geography, climate, odors, contaminants, land use, transportation costs and regulatory constraints.
Thus, the current 503 Regulations-compliant alternatives in municipal sludge disposal are: to destroy it through incineration; to land-apply it under heavy public scrutiny and an overbearing regulatory scheme; to convert it to a more desirable form through composting; or, to reduce the volume of sludge using drying methods that have heretofore been exceedingly costly. Overall, drying would be most desirable, were it not for the cost in fuel and expensive equipment.
The challenge in drying a pasty, sticky, gelatinous and difficult-to-handle material like sludge is in removing the moisture trapped inside. Typically, wet sludge cake is processed to a 20-25% solid through dewatering methods such as centrifuges and belt filter presses.
The current state of thermal-drying technology in the wastewater treatment industry is dominated by two heat drying technologies: direct and indirect. Direct drying technology puts hot air in direct contact with the biosolids during the drying process. Indirect drying technology causes the biosolids to come into direct contact with a heated surface, as opposed to hot air.
Currently there are two direct drying systems in the marketplace that employ a rotary drum, triple pass dryer, with provisions for recycling the majority of the hot air after removal of water vapor through condensation or other means: the Andritz-Ruthner DDS and the Swiss Comby system. The Andritz-Ruthner DDS is representative of all drying systems in this class. It can produce roughly 59 tons of 90%-dry biosolids per day. However, the cost for accomplishing this is high--approximately $179 per dry ton. Further, including accessories, the base cost of the Andritz-Ruthner DDS, alone, is currently approximately $8,680,000. Yet it also requires costly million-dollar scrubber systems for abatement of volatile organic compounds and nuisance odors in its exhaust stream. And, due to its complexity, exceedingly long construction times from start to finish are common.
In the category of indirect dryers, the current leader is the Komline-Sanderson (KS) dryer. The KS dryer has two rotating assemblies that convey biosolids through the dryer vessel. Each rotating assembly consists of a hollow shaft with paddles attached at regular intervals along its length. Steam is circulated through the hollow shaft and the jacketed shell of the dryer vessel to evaporate water from the biosolids being conveyed through the dryer. KS's largest dryer has a 40-ton per day capacity with a $213 per dry Ton cost.
The KS Dryer system is not appropriate for start/stop, one or two shift per day operation because of the two to three hours of time needed to start up and shut down the equipment. Further, the acquisition cost of the KS system, based upon capacity, is in excess of $10,000,000, including lengthy and costly on-site construction. Similarly, as with the Andritz-Ruthner DDS, the KS dryer requires costly, million-dollar scrubber systems for air abatement.
Dry Vac Environmental (Dry Vac) manufactures a Vacuum Filter Press (VFP) drying system, which is considered an indirect drying technology. The VFP is an adaptation of a recessed filter plate press, a conventional dewatering device which typically produces an 18%-22% dry product. The filter plate press, a batch-dewatering device, has met with limited acceptance among U.S. wastewater treatment facilities, which prefer continuous dewatering devices. In the VFP drying system, dewatering occurs by filtration as the biosolids are pumped into the filter press under pressure. Hot water is circulated through the filter press, heating the biosolids "cake" and expanding the membrane. The dried biosolids are then discharged by gravity, as the plates are mechanically separated, one filter plate at a time. Although Dry Vac's VFP drying system can produce a 90% dry product, to date, the technology has not been proven on a large-scale application. Additionally, due to the "batch" cycle of the VFP (5-6 hours), the press-emptying step and the initiation of the next operating cycle need to be monitored closely by personnel. The VFP is also handicapped by the need for frequent repair of system components. Other drawbacks of the VFP include the need for ferric chloride or lime in the solids conditioning process; these are costly and require constant housekeeping.
Another direct-drying system technology is the gas-fired rotary kiln. There are several disadvantages to this technology. First, the cost to a municipality to construct the kiln on-site is prohibitive. In 1992 the City of Tampa, Fla. spent $18 million to construct and install its gas-fired kiln. In 1994, the City of St. Petersburg, Fla. constructed its twin at a similar cost. Second, this drying process does not recoup or recycle any of its sensible and latent heat in the air stream and condensate. Thus, it is difficult, if not impossible in some states to obtain a permit to operate these systems. Third, although the gas-fired rotary kiln can dry municipal sludge to a 90%-dry product, the cost is extreme--approximately $250 per dry ton.
The Zimpro process bypasses the initial dewatering stage as the wastewater slurry is pumped into its boiler system. The boiler tank initially boils off the moisture followed by a second stage chamber where steam heats the sludge to a 90% dry product. The Zimpro steam dryer technology, like the rotary gas-fired kiln technology, is hard to permit, requires high maintenance and has a high unit cost. However, the real drawback to the Zimpro system is that it is capable of handling only small volumes of sludge.
The Hosokawa Bepex Corporation based in Minn., Minnesota owns two technologies for drying sludge slurries and cakes. The first is an established technology of pulse combustion that emerged in Germany during WWII. This drying system uses high sound levels generated by a Unison pulse combuster to enhance the drying of sludge slurries. The pulse combustion provides heat as well as motive force for atomization.
The Unison system utilizes heated air of 1200.degree.-2400.degree. F. as the drying medium creating a brief residence time. Pulse combustion is a low production system (two (2) tons/hour) at an erected and running cost of more than two (2) million dollars. Prototype work in this area has been minimal due to low production and high capital costs.
Hosokawa Bepex also manufactures a Torusdisc for heating and drying municipal sludge. Denver Screw, another competitor, makes a similar device. Both systems use heat to transfer fluids such as steam or hot oils through hollow screws within a jacketed vessel. Rotor speed controls residence time and a bone dry product results. However, these systems are low volume, and, the wear factor of the screws and the maintenance of the plumbing are high. Additionally, because of the necessary high temperature of the process heat, the organic value of the end product is destroyed.
F.D. Deskins Company's (Deskins) has a new sand dewatering and drying bed design that is quite effective to produce a 90% dry cake the texture of a potato chip. The capital cost is minimal, but the space required and the odors are prohibitive for major municipalities. The drying is accomplished through a solar/evaporative process that only works seasonally in hot climates. The Deskins system is limited to small, remote, warm climates.
Carver Greenfield is a similar technology utilizing hot oils and steams for sludge drying. This system became fashionable in the middle to late 1980's; but because its peak capacity peaked out at two (2) tons per hour (tph) feed rate, it is no longer popular.
In addition to all of the foregoing, a myriad of new wastewater treatment technologies are being developed for small-scale operation. Some of these employ ultrasonic, microwave, additional adapted plate and frame technology, and radiant heat processes. However, neither these new technologies, nor those described further above, meet the present and future high-volume sludge-processing needs of the major wastewater treatment facilities throughout the world. Indeed, virtually every wastewater treatment facility is looking for an economical, energy-efficient and environmentally-sound technology which dries municipal sludge and recycles its biosolids end product.